1. Filed of the Invention
The present invention relates to a method for oxidizing alkanes with oxygen in catalytically oxidizing reactions. In this specification, the alkanes include cycloalkanes In the present invention, the alkanes and the cycloalkanes include substituted alkanes and substituted cycloalkanes, respectively. Further, the cycloalkanes include condensed rings in which an aromatic ring is condensed with a cycloalkane ring at an ortho position.
2. Related Art Statement
In the fields of the organic chemistry and the industrial chemistry, the necessity for methods of introducing oxygen functional groups in alkanes has been increasing in recent years, and earlier development of effective oxygen-oxidizing reactions has been demanded.
Alcohols and ketones obtained by oxidizing alkanes can be used as building blocks and starting materials for medicines, chemical products and polymers as well as various functional materials.
Heretofore, an automatic oxidizing method which undergoes a free radial reaction in the presence of a cobalt-based catalyst is used as an industrial method for oxidizing alkanes with oxygen. However, such an automatic oxidizing method required high temperature and high pressure conditions to realize a sufficient oxidation speed, and further needed to suppress the conversion rate at a low level to selectively obtain intended alcohol and ketone. In order to solve such problems, an oxygen-oxidizing method which is based on a new principle with use of a gentle condition of an oxygen pressure of 1 atm has been strongly demanded.
Under the circumstances, various examinations have been effected with respect to the oxidation of hydrocarbons with oxygen with use of the gentle condition of 1 atm. For example, one equivalent or more reducing agent is used for oxidizing a hydrocarbon with oxygen at 1 atm. As the reducing agent, zinc (J. Chem. Soc., Perkin. Trans. 1 1986, 947; J. C hem. Soc., Chem. Commun. 1991, 102; New J. Chem. 16, 621 (1992)), hydrazine (J. Am. Chem. Soc. 112, 879 (1990)), hydrogen (J. Am. Chem. Soc. 103, 7371 (1981); J. Am. Chem. Soc. 109, 2837 (1987); J. Chem. Soc., Chem. Commun. 1992, 1446), hydrogen sulfide (Chem. commun. 1997, 557), ascorbic acid salt (Tetrahedron 40, 4297 (1984)), hydroquinone (Chem. Lett. 1991, 1819), etc. were proposed. Each of the above reacting systems has a problem in that the yield of the product and the turnover number of the catalyst are low.
On the other hand, the present inventors discovered a method of oxygen-oxidizing alkanes with use of aldehydes in the presence of a copper/crown ether catalyst or a copper/crown ether/inorganic salt catalyst (JP-A 6-263,664 and JP-A 11-255,682). Although this method enables the alkane to be oxidized with use of oxygen at 1 atm, it has been demanded to construct an inexpensive catalyst system having further improved catalytic activity (turnover number) and substituting for crown ethers.
The present inventors made further investigations to improve the catalytic activity with due consideration of economy, and discovered that the catalytic activity is greatly improved by coordinating a nitrogen-containing compound such as a nitrile into a copper catalyst. The inventors reached the present invention based on this discovery. That is, the present invention is to provide a method for oxidizing an alkane and a cycloalkane with oxygen in the presence of an aldehyde, a copper catalyst and a nitrogen-containing material as an auxiliary ligand. In the present claims, the alkane includes the cycloalkane.
In the following, the present invention will be described in more detail. (1) Alkanes
Specific examples of the alkanes and the cycloalkanes to be used in the oxidizing method of the present invention are given below.
As the alkanes, represented by a general formula CnH2n+2(n=1 to 30), n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, n-undecane, n-dodecane, etc. may be recited, for example. In the present invention, the alkane includes the substituted alkane represented by a general formula CnH2n+1R in which n=1 to 30, R denotes a carboxylic group, an ester group, a halogen group, a nitro group, a methoxy group, or an alkyl group or an aromatic group which may be substituted by a carboxylic group, an ester group, a halogen group, a nitro group or a methoxy group are included. As representative examples of the alkanes, mention may be made of toluene, p-xylene, m-xylene, o-xylene, 4-methoxytoluene, 3-methoxytoluene, 2-methoxytoluene, 4-chlorotoluene, 3-chlorotoluene, 2-chlorotoluene, ethylbenzene, diphenylmethane, 4-nitrotoluene, decanoic acid, methyl decanate.
The cycloalkane, which may be represented by a general formula CnH2n(n=5 to 30), and mention may be made of cyclohexane, cyclopentane, cyclooctane, cycloheptane, cyclododecane, etc. In the present invention, the cycloalkane may include a cycloalkane represented by a general formula CnH2n-1R in which n=5 to 30, R denotes a carboxylic group, an ester group, a halogen group, a nitro group, a methoxy group, or an alkyl group or an aromatic group which may be substituted by a carboxylic group, an ester group, a halogen group, a nitro group or a methoxy group. As examples of the cycloalkanes, mention may be made of methylcyclohexane, adamantane, cis-decalin, trans-decalin, cyclohexane carboxylic acid, cyclohexane carboxylic acid, methylchlorocyclohexane, nitrocyclohexane, etc. The cycloalkanes include a condensed ring in which an aromatic ring and a cycloalkane ring are condensed at an ortho position. As examples thereof, indane, 5, 6-dimethoxyindane, tetralin, fluorene, etc. may be recited.
As copper-based catalysts to be used in the present invention, conventionally known inorganic copper salts may be employed. For example, Cu(OAc)2-nH2O, Cu(OAc), Cu(OCOCF3)2, CuCN, CuCl, CUCl2-nH2O, CuBr, CuBr2, CuSO4-nH2O, Cu(NO3)2-nH2O, Cu(ClO4)2, Cu(OCH3)2, Cu(PO4)2-nH2O, CuO, Cu2O, Cu(acac)2, Cu(OH)2, Cu powder, etc. in which n is an integer of 0 to 6. In addition, copper coordination compounds which is preliminarily replaced by a nitrogen-series ligand, and peroxo crosslinked binuclear copper complexes easily formed therefrom in the presence of oxygen may be employed. For example, mention may be made of X, X (C5H5N: pyridine), X (bpy=2, 2xe2x80x2-bipyridine), X2 (C3H4N2: imidazole), X2 (phen=1, 10-phenanthroline), X2 (C14H32N4=1, 4, 8, 11-tetramethyl-1, 4, 8, 11-tetraazacyclotetradecane), X (tmpa =tris(2-pyridylmethyl)amine), X, X2, X in which X, X are Cl, NO3, ClO4, PF6, BF4 or the like. Among them, Cu(OAc)2 which is easily available and handled and has high reactivity is preferably used. The use amount of the copper-based catalyst is not particularly limited, but it is ordinarily in a range of 0.000001 to 200 mol %, preferably in a range of 0.00001 to 5 mol % relative to a substrate.
As the aldehydes to be used in the present invention, aliphatic aldehydes and aromatic aldehydes may be employed. More specifically, mention may be made of aliphatic aldehydes such as acetaldehyde, propionaldehyde, n-butylaldehyde, isobutylaldehyde, n-valeraldehyde, isovaleraldehyde, pivalaldehyde, n-hexylaldehyde, n-heptylaldehyde, n-octylaldehyde, n-nonylaldehyde, n-decylaldehyde, etc., substituted or non-substituted aromatic aldehydes such as benzaldehyde, p-chlorobenzaldehyde, m-chlorobenzaldehyde, m-cyanobenzaldehyde, p-tolualdehyde, p-methoxybenzaldehyde, etc, and mixtures of the above aldehydes. Among them, acetaldehyde which is easily industrially available is preferably employed from the standpoint of reactivity and economy. The use amount of the aldehyde is not particularly limited, but it is ordinarily in a range of 0.1 to 1000 mol %, preferably in a range of 1 to 400 mol % relative to the substrate.
The nitrogen-containing compounds to be used in the present invention may be nitrites, aromatic amines and tertiary amines. As the nitrites, conventionally known nitrites may be used, irrespective of aliphatic nitrites or aromatic nitrites. As specific examples of the aliphatic nitrites, mention may be made of acetonitrile, propionitrile, butylonitrile, isobutylonitrile, valeronitrile, isovaleronitrile, trimethylacetonitrile, hexane nitrile, 4-methylvaleronitrile, heptane nitrile, octane nitrile, undecane nitrile, decane nitrile, stearonitrile, cyclohexane carbonitrile, cyclopentane carbonitrile, etc. As aliphatic dinitriles, mention may be made of malononitrile, succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, suberonitrile, azelanitrile, etc. As the aromatic nitrites, benzonitrile, o-, m-, p-phthalonitriles, etc. may be recited. As specific examples of the aromatic amines and the tertiary amines, mention may be made of non-substituted or substituted amines or derivatives such as pyridine, 2, 2 bipyridine, 2, 2 biquinoline (biquinoline), 2, 2:6 2 terpyridine (terpyridine), imidazole, pyrazole, 1, 10-phenanthroline, 1, 4, 8, 11-tetramethyl-1, 4, 8, 11-tetraazacyclotetradecane, tris(2-pyridylmethyl)amine, etc. Among them, acetonitrile is preferably employed from the standpoint of reactivity and economy. The amount of the nitrile, the aromatic amine or the tertiary amine is not particularly limited, but it is ordinarily in a range of 0.000001 to 50 equivalents relative to the substrate. The amount of the nitrile is preferably in a range of 1 to 2 equivalents relative to the substrate.
(5-1) Solvents
With respect to the solvent to be used in the present reaction, a hydrocarbon (for example, cyclohexane) as a substrate may be used as functioning as both a substrate and a solvent. Further, another solvent may be used. As the solvent, use may be made of halogenated hydrocarbons (methylene chloride, chloroform, etc.), ketones (acetone, etc.), esters (ethyl acetate, etc.), carboxylic acids (acetic acid, etc.), aromatic hydrocarbons (benzene, chlorobenzene, etc.), etc. They may be used singly or in a mixed state of two or more of them.
(5-2) Reacting temperature
The reacting temperature is ordinarily in a range of 0xc2x0 C. to 150xc2x0 C., preferably in a range of 25xc2x0 C. to 80xc2x0 C. Although the reaction proceeds when the reacting temperature is room temperature such as 20xc2x0 C., about 10 to 100 times as much as the catalyst is necessary, and the reaction time becomes longer in some cases.
(5-3) Other treatments
After the reaction solution is treated with an aqueous solution of sodium hydrogensulfite, the catalyst can be removed from an alcohol or a ketone produced corresponding to the alkane or the cycloalkane in the present reaction by a treatment such as washing with water. The alcohol or ketone can be easily separated from the reaction mixture by an operation such as distillation, if necessary.
A method for carrying out the oxygen-oxidizing reaction will be explained.
As oxygen used in the present reaction, any one of (a) oxygen gas, (b) air (oxygen 20%, nitrogen 78%, carbon dioxide 1%, etc.), (c) a mixture of oxygen gas and an inert gas such as nitrogen may be used. The pressure of the gas may be a reduced pressure or a pressurized pressure, but the oxygen pressure may be 1 atm.
It is preferable that the reaction is effected in an experiment at around room temperature in the state that a reacting container is attached with an oxygen-filled balloon for supplying oxygen at 1 atm.
In a high-temperature experiment at 40xc2x0 C. or more, an autoclave made of Hastelloy is preferably used. The reaction in the experiment is preferably effected such that the container is first replaced with nitrogen at 8 atm, and the autoclave is connected to an apparatus to which oxygen is always fed at 1 atm starting from the reaction start and during the reaction.
Further, the aldehyde to be used in the reaction is purified, if necessary (For example, aldehyde is used as being distilled).
The method of oxidizing the alkanes and the cycloalkanes according to the present invention is considered as follows. For example, when acetoaldehyde and a divalent copper catalyst are taken as an example, acetoaldehyde generates acyl radicals in the presence of the divalent copper catalyst, which produces a peracetic acid with oxygen through an automatic oxidation process. At this time, the divalent copper is reduced to monovalent copper. The resulting monovalent copper reacts with peracetic acid to form a trivalent copper active species, which extracts a hydrogen atom from the alkane. Following this, an alcohol is produced through recoupling of a hydroxyl ligand, and monovalent copper is reproduced. The alcohol is oxidized in a similar way.